(1) Field of the Invention
The present invention relates to transparent resins having high surface hardness and excellent heat resistance, that are suitable for applications such as glazing materials, optical information recording media and optical lenses.
(2) Description of the Prior Art
Methacrylic resins and polycarbonate resins have excellent transparency and mechanical properties, and are accordingly widely used as covers for light fixtures and signboards, and vehicle windows. Recently, the potential use of plastic lenses and substrates for optical disks has also been discussed. However, since these resins are linear polymers, their surface hardness and chemical resistance are not necessarily sufficient for these applications. Moreover, if the surface of the resin is treated with a hard coating agent, the selection of the type of the solvent that can be used is quite limited. Accordingly, in these respects, improvement in these types of resins is eagerly awaited.
To improve the surface hardness and the chemical resistance of methyl methacrylate resins, various methods have been proposed wherein a monomer having polymerizable functional groups is added to methyl methacrylate, and copolymerization is carried out to cause crosslinking to occur. However, a disadvantage of these methods is that the volume decrease of methyl methacrylate itself is high (about 21%) when polymerized, and the crosslinking reaction proceeds simultaneously with the onset of polymerization. This means that pre-polymerization processes cannot be performed. That is, in the early stages of polymerization, since the viscosity increases sharply, the mixture loses fluidity, and it becomes difficult to transfer the liquid for injection. Thus, in these methods, casting polymerization must be carried out without pre-polymerization, and therefore the problem of volume decrease, or shrinkage, cannot be solved.
Crosslinked resins produced by polymerizing diethylene glycol diallyl carbonate are widely known for their excellent surface hardness, but the volume shrinkage thereof is also high (about 14%) when polymerized, and it takes a long time before polymerization is complete. Again this cannot be solved by pre-polymerization, because of problems similar to those encountered with the above methods.
In order to solve these problems, urethane polyacrylic esters and urethane polymethacrylic esters have been proposed (Japanese Laid-Open Patent Application Nos. 3610/1986, and 75022/1988). However, since these monomers, like those discussed above, have only an acryl group or a methacryl group in the molecule, whose polymerization rate is similar, a crosslinking reaction also proceeds when free radical polymerization is effected. Consequently, pre-polymerization is difficult to carry out, and polymerization is hardly controlled, and takes place rapidly, with the likely result that the resultant polymer will separate from the mold and have a rough surface.